Conceiving reliablesystems is a strategic issue for any industrial society. Hence reliability
has become a discipline at the beginning of the Second World War. In fact reliability is a
field of reseach common to mathematics operational research informatics graph theory
physics and so forth. We are concerned here with the mathematical side of reliability of
which probability statistics and more specially stochastic processes theory constitute the
natural basis. US army during the war and later in the US Problems encountered by the and
Soviet space programs have led to an awarenessofthe need for reliabilityor more generaly for
dependability (a general term covering reliability availability security maintainability
etc.) of the systems. The paper by W. Weibull of 1938 on the strength of materials leading to
the distribution that later took his name and the paper by B. Epstein and M. Sobel of 1951
initiating the use of the exponential distribution as the basic (and now most used) model for
reliability are the founding papers of the field. At this time the systems were merely seen
as black boxes. During the 1960s they began to be considered as the result of the interaction
of their elements. Appropriate methods were then developed from Shannon's work to the
beautiful theory of coherent systems initiated by Z.W. Birnbaum J.D.
